Rectifier circuit and RFID tag

ABSTRACT

A rectifier circuit includes a first MOS transistor, a first capacitor configured to connect between a gate and a source of the first MOS transistor, and a switching circuit configured to supply a bias voltage to the first capacitor in response to a control signal. The high gain rectifier circuit also includes a second MOS transistor configured to imitate the first MOS transistor, a second capacitor configured to imitate the first capacitor, a dummy switching circuit configured to supply the bias voltage to the second capacitor in response to the control signal, and a generating circuit configured to generate the control signal based on a potential of the second capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2004-325330, filed on Nov. 9,2004; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rectifier circuit including adiode-connected MOS transistor with a capacitor connected between thegate and source, and a radio frequency identification (RFID) tagincluding the rectifier circuit.

2. Description of the Related Art

A rectifier circuit converts alternating current (AC) into directcurrent (DC) through the rectification of diodes. The rectifier circuit,when is provided as a semiconductor integrated circuit, employs adiode-connected MOS transistor whose source and gate are connected toeach other as a rectifier diode. The diode-connected MOS transistor is,for example, an NMOS transistor isolated from a substrate through atriple well where the drain and source are connected to an N-well andthe source is connected to the back gate connected to a P-well locatedat the bottom of the transistor. In this NMOS transistor, a diode isprovided as a PN junction formed between the source and drain.

An RFID tag, which is recently watched because of its wide application,requires a rectifier circuit. The RFID tag generates a direct-currentpower-supply voltage for driving the integrated circuit in the RFID tag,and demodulates data signals, from an alternating current induced in aloop antenna. The rectifier circuit serves for the voltage generationand demodulation.

Such a rectifier circuit used in the RFID tag is proposed in, forexample, Japanese Patent Application Laid-Open No. 2002-152080 and M.Usami et al., “Powder LSI: An ultra small RF identification chip forindividual recognition applications,” ISSCC Dig. Tech. Papers, February2003, pp. 398-399. According to the proposed rectifier circuit, if theMOS transistor is lower in threshold voltage than the PN junction, therectification properties of the diode-connected MOS transistor dependson the properties of the MOS transistor, and accordingly isapproximately the same as the rectification properties of a diodeincluding a MOS transistor whose threshold voltage is equal to thethreshold voltage of a PN junction.

However, to perform rectification of the diode, a voltage not less thanthe threshold voltage of the PIN junction or the threshold voltage ofthe MOS transistor must be applied across the PN junction, i.e. acrossthe source and drain. The voltage to be applied across the PN junctioncan be supplied from, for example, a capacitor connected between thegate and source of the MOS transistor and retaining a voltage from zeroto the threshold voltage (hereinafter referred as to “bias voltage”).Hence, the rectifier circuit, even if receiving an AC signal with aroot-mean-square value of less than the bias voltage, can rectify such alow signal. This means that the RFID tag can receive a weak signaltransmitted by a tag reader or writer and the communication rangebetween the RFID tag and the tag reader or writer becomes wider. Thewider communication range makes one reader or writer easy to detectplural RFID tags simultaneously, and widens the application range of theRFID tag.

However, since the electric charges stored in the capacitor isdischarged through the leakage current of the MOS transistor to whichthe capacitor is connected, the voltage across the capacitor decreaseswith time. This means the AC signal that can be rectified increases. Inother words, the conversion gain of the rectifier circuit decreases. Tokeep the conversion gain high, a control signal for charging thecapacitor should be periodically transmitted to the rectifier circuit.If the control signal is periodically generated using a counter,charging the capacitor will be needlessly repeated. Such overcharge isundesirable because low power consumption is an important design factorof electronic devices such as the RFID tag.

The bias voltage is preferably almost the threshold voltage of the MOStransistor to be biased. This is because more than the threshold voltagecauses the current in the MOS transistor to flow backward, so that thegain of the rectifier circuit decreases. However, even if the biasvoltage is fixed to a preset value, the gain of the rectifier circuitmay become small due to manufacturing differences of the MOS transistor.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a rectifier circuitincludes a first MOS transistor; a first capacitor configured to connectbetween a gate and a source of the first MOS transistor; a switchingcircuit configured to supply a bias voltage to the first capacitor inresponse to a control signal; a second MOS transistor configured toimitate the first MOS transistor; a second capacitor configured toimitate the first capacitor; a dummy switching circuit configured tosupply the bias voltage to the second capacitor in response to thecontrol signal; and a generating circuit configured to generate thecontrol signal based on a potential of the second capacitor.

According to another aspect of the present invention, a radio frequencyidentification tag includes an antenna; a rectifier circuit; a batterythat is charged with a direct current supplied from the rectifiercircuit; and a control circuit that transmits tag identificationinformation via the antenna based on the direct current. The rectifiercircuit includes a rectifier circuit that includes a first MOStransistor serving as a rectifier, a first capacitor connected between agate and a source of the first MOS transistor; and a switching circuitfor supplying a bias voltage to the first capacitor in response to acontrol signal. The rectifier circuit also includes a bias settingcircuit that includes a second MOS transistor imitating the first MOStransistor, a second capacitor imitating the first capacitor, and adummy switching circuit for supplying the bias voltage to the secondcapacitor in response to the control signal, and generates the controlsignal based on a potential of the second capacitor.

According to still another aspect of the present invention, a radiofrequency identification tag includes an antenna;

a first MOS transistor;

a first capacitor configured to connect between a gate and a source ofthe first MOS transistor; a switching circuit configured to supply abias voltage to the first capacitor in response to a control signal; asecond MOS transistor configured to imitate the first MOS transistor; asecond capacitor configured to imitate the first capacitor; a dummyswitching circuit configured to supply the bias voltage to the secondcapacitor in response to the control signal; generating unit configuredto generate the control signal based on a potential of the secondcapacitor; a battery configured to be charged with a direct currentsupplied from the rectifier circuit; and a control circuit configured totransmit tag identification information via the antenna based on thedirect current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a rectifier circuit according to a firstembodiment;

FIG. 2 is a circuit diagram of a differential amplifier;

FIGS. 3A to 3D are circuit diagrams showing various examples of a biasvoltage source or a reference voltage source;

FIG. 4 is a timing chart of a bias setting circuit of the rectifiercircuit according to the first embodiment;

FIG. 5 is a circuit diagram of a bias setting circuit of a rectifiercircuit according to a second embodiment;

FIG. 6 is a timing chart of the bias setting circuit of the rectifiercircuit according to the second embodiment;

FIG. 7 is a circuit diagram of a rectifier circuit of the rectifiercircuit according to the second embodiment;

FIG. 8 is a circuit diagram of a power detector circuit generating areset signal;

FIG. 9 is a timing chart of the power detector circuit of the rectifiercircuit according to the second embodiment;

FIG. 10 is a circuit diagram of a cascade rectifier circuit of arectifier circuit according to a third embodiment;

FIG. 11 is a detailed circuit diagram of a first rectifier circuit stageof the cascade rectifier circuit in FIG. 10;

FIG. 12 is a circuit diagram of a bias setting circuit constituting therectifier circuit according to the third embodiment;

FIG. 13 is a circuit diagram of a bias setting circuit constituting arectifier circuit according to a fourth embodiment;

FIG. 14 is a timing chart of the bias setting circuit of the rectifiercircuit according to a fourth embodiment;

FIG. 15 is a circuit diagram of a bias setting circuit constituting arectifier circuit according to a fifth embodiment;

FIG. 16 is a circuit diagram of a bias setting circuit constituting arectifier circuit according to a sixth embodiment;

FIG. 17 is a block diagram of an RFID tag according to a seventhembodiment;

FIG. 18 is a timing chart of a reset signal;

FIG. 19 is a block diagram of another RFID tag according to the seventhembodiment; and

FIG. 20 is a block diagram of an RFID tag with a sensor according to theembodiments.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of a rectifier circuit and an RFID tag includingthe rectifier circuit according to the present invention will be nowdescribed in detail with reference to the accompanying drawings.

A rectifier circuit according to a first embodiment of the presentinvention includes a dummy MOS transistor that imitates a MOS transistor(hereafter, referred to as “rectifier MOS transistor”) serving as arectifier. The rectifier circuit also monitors a potential of acapacitor connected between the gate and source of the dummy MOStransistor to charge a capacitor connected to the rectifier MOStransistor at the perfect time.

FIG. 1 is a circuit diagram of the rectifier circuit according to thefirst embodiment. The rectifier circuit according to the firstembodiment includes a rectifier circuit 200 and a bias setting circuit100 as shown in FIG. 1.

The rectifier circuit 200 includes a rectification circuit forrectifying a received AC signal, and a switching circuit for applying apredetermined potential to a capacitor in rectification circuit.

The rectification circuit is composed of two NMOS transistors M1 and M2connected in series. The NMOS transistor M1 has a back gate and a sourcewhich are connected to each other, and a drain connected to a positiveterminal T1. A capacitor C12 is connected between the gate and source ofthe NMOS transistor M1. This connection makes the NMOS transistor M1function as a diode because the gate of the NMOS transistor M1 is biasedwith a voltage across the capacitor.

The NMOS transistor M2 has a back gate and a source which are connectedto each other. The source of the NMOS transistor M2 is connected to anegative terminal T2. A capacitor C22 is connected between the gate andsource of the NMOS transistor M2. The NMOS transistor M2 functions alsoas a rectifier like the NMOS transistor M1, and is biased with a voltageacross the capacitor C22.

The source of the NMOS transistor M1 and the drain of the NMOStransistor M2 are connected to each other, and a line connecting them isconnected to one end of a capacitor C1. The other end of the capacitorC1 is connected to a signal input terminal TA to which an AC signal isinput. The capacitor C1 functions as a coupling capacitor. When therectifier circuit according to this embodiment is used in an RFID tag,the capacitor C1 is connected to an antenna (e.g., a loop antenna) andfunctions as a series resonance capacitor.

A capacitor C2 is connected between the drain of the NMOS transistor M1and the source of the NMOS transistor M2, and smoothes the signalhalf-wave rectified by the NMOS transistors M1 and M2. This smoothingallows the output of a DC voltage from both ends of the capacitor C2,that is, between the positive terminal T1 and the negative terminal T2.

The NMOS transistors M1 and M2 are formed as a triple well structure andare isolated from a substrate. Each source is connected to a p-welllocated at the bottom of the NMOS transistor, and each drain isconnected to an n-well. A diode is thus formed as a PN junction in eachMOS transistor.

The switching, circuit transfers the voltage supplied from the biasvoltage source 210 to the capacitors C12 and C22 according to thecontrol signal V1. The voltage supplied to the capacitors C12 and C22 isa voltage (hereinafter referred to as “bias voltage V_(T)”) less than athreshold required for the rectification in the NMOS transistors M1 andM2, for example, ranging from 0 to 1.0 V. The bias voltage V_(T) ispreferably almost the threshold voltage (e.g., 0.6 V). As a result, theNMOS transistors M1 and M2, which constitute a diode circuit, canrectifier an AC signal input to the signal input terminal TA even if theAC signal has a root-mean-square value of less than the thresholdvoltage. When the bias voltage V_(T) is, for example, 0.6 V, the diodecircuit can rectify an AC signal with a root-mean-square value ofapproximately 100 mV.

The switching circuit is composed of a plurality of NMOS transistors M11to M14 and M21 to M24, which function as transfer gates, inverters INV1and INV2, and the bias voltage source 210. The NMOS transistors M11 andM12 are connected in series and are arranged in a first positive lineconnected to the positive terminal of the bias voltage source 210. Thefirst positive line is connected from the positive terminal of the biasvoltage source 210 to the gate of the NMOS transistor M1 and the one endof the capacitor C12 through the NMOS transistors M12 and M11. The NMOStransistors M13 and M14 are connected in series and are arranged in afirst negative line connected to the negative terminal of the biasvoltage source 210. The first negative line is connected from thenegative terminal of the bias voltage source 210 to the source of theNMOS transistor M1 and the other end of the capacitor C12 through theNMOS transistors M14 and M13. A capacitor C11 is connected between aline connecting the source of the NMOS transistor M11 to the drain ofthe NMOS transistor M12 and a line connecting the source of the NMOStransistor M13 to the drain of the NMOS transistor M14. In other words,the capacitor C11 is connected between the first positive line and thefirst negative line.

The NMOS transistors M21 and M22 are connected in series and arearranged in a second positive line connected to the positive terminal ofthe bias voltage source 210. The second positive line is connected fromthe positive terminal of the bias voltage source 210 to the gate of theNMOS transistor M2 and the one end of the capacitor C22 through the NMOStransistors M22 and M21. The NMOS transistors M23 and M24 are connectedin series and are arranged in a second negative line connected to thenegative terminal of the bias voltage source 210. The second negativeline is connected to the source of the NMOS transistor M2 and the otherend of the capacitor C22 through the NMOS transistors M24 and M23. Acapacitor C21 is connected between a line connecting the source of theNMOS transistor M21 to the drain of the NMOS transistor M22 and a lineconnecting the source of the NMOS transistor M23 to the drain of theNMOS transistor M24. In other words, the capacitor C21 is connectedbetween the second positive line and the second negative line.

Each gate of the NMOS transistors M11, M13, M21, and M23 is connected tothe output terminal of the inverter INV1. Each gate of the NMOStransistors M12, M14, M22, and M24 is connected to the input terminal ofthe inverter INV1. The input terminal of the inverter INV1 is connectedto the output terminal of the inverter INV2.

This switching circuit operates as follows. The NMOS transistors M12,M14, M22, and M24 are ON during a period when the control signal V1input to the inverter INV2 is a logic “low”, so that the capacitors C11and C21 are charged to the bias voltage V_(T) with the bias voltagesource 210. Meanwhile, the output of the inverter INV1 is a logic “low”,so that the NMOS transistors M11, M13, M21, and M23 are OFF, andtherefore neither capacitor C12 nor C22 is charged. The NMOS transistorsM12, M14, M22, and M24 are OFF and the NMOS transistors M11, M13, M21,and M23 are ON during a period when the control signal V1 is a logic“high”, so that the charges in the capacitor C11 are transferred to thecapacitor C12 and the charges in the capacitor C21 are transferred tothe capacitor C22. Consequently, the bias voltage V_(T) is appliedbetween the gate and source of the NMOS transistor M1 and between thegate and source of the NMOS transistor M2. The control signal V1 inputto the inverter INV2 of the rectifier circuit 200 is generated with thebias setting circuit 100. The bias setting circuit 100 includes a dummyrectification circuit, a dummy switching circuit, and a control signalgeneration circuit. The dummy rectification circuit is apartially-duplicated circuit of the rectification circuit of therectifier circuit 200 and is composed of an NMOS transistor M30 havingthe same characteristics as that of the NMOS transistor M1 or M2, and acapacitor C32 corresponding to the capacitor C12 or C22. The NMOStransistor M30 has a back gate and a source, which are connected to eachother, and a drain connected to the source and the ground. A capacitorC32 is connected between the gate and source of the NMOS transistor M30.In other words, the dummy rectification circuit imitates one of therectifiers that constitute the rectification circuit of the rectifiercircuit 200.

The dummy switching circuit has a circuit corresponding to the circuitfor supplying the bias voltage V_(T) to the capacitor C12 or C22, out ofthe switching circuit of the rectifier circuit 200. Referring to FIG. 1,the dummy switching circuit corresponds to a circuit composed of NMOStransistors M31 to M34, a capacitor C31, and inverters INV11 and INV12.

Specifically, the NMOS transistors M31 and M32 are connected in seriesand are arranged in a positive line connected to the positive terminalof the bias voltage source 110. This positive line is connected to thegate of the NMOS transistor M30 and one end of the capacitor C32 throughthe NMOS transistors M32 and M31. The NMOS transistors M33 and M34 areconnected in series and are arranged in a negative line connected to thenegative terminal of the bias voltage source 110. This negative line isconnected to the source of the NMOS transistor M30 and the other end ofthe capacitor C32 through the NMOS transistors M34 and M33. A capacitorC31 is connected between a line connecting the source of the NMOStransistor M31 to the drain of the NMOS transistor M32 and a lineconnecting the source of the NMOS transistor M33 to the drain of theNMOS transistor M34. Each gate of the NMOS transistors M31 and M33 isconnected to the input terminal of the inverter INV11. Each gate of theNMOS transistors M32 and M34 is connected to the output terminal of theinverter INV11. This dummy switching circuit corresponds to the circuitfor supplying the bias voltage V_(T) to the capacitor C12 (C22): theNMOS transistors M11 to M14 (M21 to M24), the capacitor C11 (C21), theinverters INV1 and INV2, and the bias voltage source 210 in therectifier circuit 200. The bias voltage source 110 supplies the biasvoltage V_(T) like the bias voltage source 210.

The control signal generation circuit monitors the potential of thecapacitor C32 in the dummy rectification circuit and generates thecontrol signal V1 based on a result of the monitor. Referring to FIG. 1,the control signal generation circuit corresponds to a circuit composedof the inverter INV12, a reference voltage source 120, and adifferential amplifier 130. The differential amplifier 130 has anon-inverting input terminal connected to the gate of the NMOStransistor M30 in the dummy rectification circuit and an inverting inputterminal connected to the positive terminal of the reference voltagesource 120. The non-inverting input terminal of the differentialamplifier 130 is specifically connected to one end of the capacitor C32(in the positive line). Hereinafter, the potential at the one end of thecapacitor C32 is referred to as V₀. The reference voltage source 120generates a reference voltage V_(T)−V_(X). The voltage V_(X) will beexplained later. In the circuitry as described above, the differentialamplifier 130 outputs a differential voltage V_(E) corresponding to adifference value calculated by subtracting the reference voltageV_(T)−V_(X) from the potential V₀ at the one end of the capacitor C32.

FIG. 2 is a circuit diagram of an example of the differential amplifier130. The differential amplifier shown in FIG. 2 is composed of PMOStransistors M121 and M122, which function as load transistors, NMOStransistors M123 and M124, which is provided as a differential pair, anda constant-current source 121. The PMOS transistors M121 and M122provide a positive feedback by the connection of their gate and drain.The gate of the NMOS transistor M124 corresponds to the non-invertinginput terminal IN1 of the differential amplifier 130, and the gate ofthe NMOS transistor M123 corresponds to the inverting input terminal IN2of the differential amplifier 130 in FIG. 2. The output terminal OUTconnected to the drain of the NMOS transistor M124 corresponds to theoutput terminal of the differential amplifier 130. The differentialamplifiers 130 may be composed of a circuitry other than that shown inFIG. 2, for example, a circuitry having a current mirror pair of loadtransistors without a positive feedback.

The output terminal of the differential amplifier 130 is connected tothe input terminal of the inverter INV12. As a result, the inverterINV12 outputs a logic “low” when the differential voltage V_(E) outputfrom the differential amplifier 130 reaches not less than apredetermined level, and the inverter INV12 outputs a logic “high” whenthe differential voltage V_(E) drops to less than a predetermined level.The signal output from the inverter INV12 is the control signal V₁. Inother words, the control signal generation circuit outputs the controlsignal V₁ of logic “low” to the rectifier circuit 200 if the potentialof the capacitor C32 in the dummy rectification circuit is not less thana potential calculated by subtracting the voltage V_(X) from the biasvoltage V_(T). The output terminal of the inverter INV12 is connected tothe input terminal of the inverter INV11, and therefore the controlsignal V1 is also input to the dummy switching circuit. As a result, thedummy rectification circuit and the dummy switching circuit imitate therectification circuit and the operation of the switching circuit of therectifier circuit 200, respectively.

As described above, in the rectifier circuit 200 and the bias settingcircuit 100, the bias voltage source 210 and the bias voltage source 110must generate the bias voltage V_(T), which is a constant voltage,respectively. However, there is a possibility that the bias voltageV_(T) does not indicate a desired value because of the manufacturingdifferences of the electronic devices that constitute the bias voltagesource. The same is true for the reference voltage source 120.

FIGS. 3A to 3D are circuit diagrams showing various examples of a biasvoltage source or a reference voltage source used in the rectifiercircuit according to the embodiment. The voltage source shown in FIG. 3Ais composed of an NMOS transistor M101 whose gate and drain areconnected to each other, and a current source 111 that generates a weakcurrent I_(BB) from a power supply voltage V_(DD). The output terminalT100 is connected to the drain of the NMOS transistor M101, and thegate-to-source voltage of the NMOS transistor M101 generated from thecurrent I_(BB) is output therefrom. Since the gate-to-source voltage isapproximately the same as the threshold voltage of the NMOS transistorM101, the voltage source can supply the gate-to-source voltage as thebias voltage V_(T) and thus be used as the bias voltage source 210 andthe bias voltage source 110. This is on the basis of the theory that ingeneral the characteristic of a MOS transistor is represented byI_(D)=β(V_(GS)−V_(th))², and the low current I_(D) makes the voltageV_(GS) between the gate and source almost equal to the threshold voltageV_(th).

Since the NMOS transistor M101 is taken out of a semiconductor waferwhere the NMOS transistors M1 and M2 of the rectifier circuit 200 andthe NMOS transistor M30 of the bias setting circuit 100 are formed, theNMOS transistor M101 has almost the same characteristic as these NMOStransistors M1, M2, and M30. The bias voltage V_(T) to be generated isapproximately the same as the threshold voltage of the NMOS transistorsM1 and M2 that constitute the rectification circuit of the rectifiercircuit 200 in at least the same rectifier circuit. In other words, thebias voltage V_(T) need not be set as an absolute value, and theoperation of the rectifier circuit is not influenced by themanufacturing differences among the rectifier circuits. Moreover, if β,which is a scale factor, is larger, the gate-to-source voltage V_(GS)can be approximately the same as the reference voltage V_(T)−V_(X) touse the voltage source shown in FIG. 3A as the reference voltage source120. Here, the voltage V_(X) is, for example, 50 mV or less.

The voltage source shown in FIG. 3B is composed of two NMOS transistorsM111 and M112, each of which has a gate and drain connected to eachother, and the current source 111 that generates a weak current I_(BB)as that in FIG. 3A. The NMOS transistors M111 and M112 are connected inseries. The output terminal T100 is connected to the drain of the NMOStransistor M112, and the sum of the gate-to-source voltage of the NMOStransistor M112 generated by the, current I_(BB) and the gate-to-sourcevoltage of the NMOS transistor M111 is output therefrom. Each thresholdvoltage of the NMOS transistors M111 and M112 is smaller than eachthreshold voltage of the NMOS transistors M1 and M2 of the rectifiercircuit 200 and the threshold voltage of the NMOS transistor M30 of thebias setting circuit 100, and is a value such that the sum of thegate-to-source voltages becomes equal to the bias voltage V_(T) or thereference voltage V_(T)−V_(X). Thus, the voltage source can be used asthe bias voltage sources 210 and 110 or the reference voltage source 120without influence of the manufacturing differences even if the voltagesource that includes a plurality of the MOS transistors each having athreshold lower than that of any of the NMOS transistors M1, M2, and M30is used. It should be noted that since their threshold voltages are notthe same, the manufacturing differences is distinct.

The voltage source shown in FIG. 3C is an example of another voltagesource that includes a first output terminal T102 connected to the drainof the NMOS transistor M112, and a second output terminal T101 connectedto the drain of the NMOS transistor M111 in the voltage source shown inFIG. 3B. In this example, the bias voltage V_(T) is output from thefirst output terminal T102, and the reference voltage V_(T)−V_(X) isoutput from the second output terminal T101, by adjusting β and thethreshold voltage of the NMOS transistors M111 and M112.

In the voltage source shown in FIG. 3D, the current of current source111 flows through an NMOS transistor M113 and resistors R1 and R2. Thisis another example of the circuit in which the bias voltage V_(T) isoutput from the first output terminal T102 and the reference voltageV_(T)−V_(X) is output from the second output terminal T101. Thegate-to-source voltage of the NMOS transistor M113, i.e., the biasvoltage V_(T) is applied across the resistors R1 and R2 because of thelow current flowing through the NMOS transistor M113. From the secondoutput terminal T101, R2/(R2+R1) times the bias voltage V_(T) is output.Adjusting the resistances of the resistors R1 and R2 can generate thevoltage of V_(T)−V_(X).

The operation of the bias setting circuit 100 will be described below.FIG. 4 is a timing chart of the potential V₀ at one end of the capacitorC32, the differential voltage V_(E) output from the differentialamplifier 130, the control signal V₁ of the inverter INV12, and a signalV2 output from the inverter INV11.

The differential amplifier 130 outputs the positive differential voltageV_(E) saturated to a predetermined value during a time period when thepotential V₀ is more than the reference voltage V_(T)−V_(X), i.e. untiltime to (the first phase). This positive differential voltage V_(E) is alogic “high” for the inverter INV12. Consequently, during the timeperiod, the control signal V₁ of the inverter INV12 is a logic “low” andthe signal V₂ of the inverter INV11 is a logic “high”. As a result, theNMOS transistors M32 and M34 are turned ON, and the bias voltage V_(T)of the bias voltage source 110 is applied to the capacitor C31, so thatthe potential of the capacitor C31 is the bias voltage V_(T).

Since the capacitor C32 is discharged through leakage current of theNMOS transistor M30, the potential V₀ decreases gradually and finallybecomes smaller than the reference voltage V_(T)−V_(X) (the secondphase). Specifically, the differential voltage V_(E) output from thedifferential amplifier 130 decreases gradually from the positivesaturated level, and finally becomes an input signal of logic “low” forthe inverter INV12 (time t₁: the third phase). As a result, the controlsignal V₁ of the inverter INV12 becomes a logic “high”, and thepotential V₂ of the inverter INV12 becomes a logic “low”. Moreover, theNMOS transistors M31 and M33 are turned ON, and the potential V_(T) ofthe capacitor C31 is applied to the capacitor C32. Specifically, thepotential V₀ of the capacitor C32 is almost equal to the potential V_(T)more than the reference voltage V_(T)−V_(X), so that there becomes thefirst phase. After that, the first to third phases are repeated.

The control signal V₁ is a pulse generated periodically during the phaserepetition. The rectifier circuit 200 operates in synchronization withthe control signal V₁, and the dummy rectification circuit and the dummyswitching circuit of the bias setting circuit 100 imitate therectification circuit and the switching circuit of the rectifier circuit200, respectively. Therefore, it is possible to charge the capacitorsC12 and C22 of the rectification circuit in the rectifier circuit 200with the bias setting circuit 100 without waste at the perfect time, andthus to constantly bias the NMOS transistors M1 and M2 that constitutethe rectification circuit to the voltage more than a predetermined valueat any time. In other words, the gain of the rectifier circuit 200 canbe maintained at more than a predetermined level at any time.

Though the circuits as described above employs the NMOS transistors,PMOS transistors may be used instead.

According to the rectifier circuit of the first embodiment, thecapacitors C12 and C22 are monitored through the bias setting circuit100, and the control signal V₁ for determining the time of charging thecapacitors C12 and C22 is generated based on a result of the monitor. Asa result, it is possible to prevent overcharge and thus to reduce powerconsumption.

Moreover, since the bias voltage V_(T) applied to the capacitors C12 andC22 and the reference voltage V_(T)−V_(X) used in the bias settingcircuit 100 are generated from the threshold voltage of another NMOStransistor designed based on the characteristics of the NMOS transistorsM1 and M2, the bias voltage V_(T) and the reference voltage V_(T)−V_(X)are not influenced by the manufacturing differences of the rectifiercircuits, and thus the rectifier circuit can have a more than desiredgain at any time.

A rectifier circuit according to a second embodiment differs from therectifier circuit according to the first embodiment in that aninitialization function is added.

There is a possibility that the potential of the capacitor C32 is muchlower than the reference voltage V_(T)−V_(x) in the bias setting circuit100 shown in FIG. 1, in an initial state of the rectifier circuit. Inthis state, the potential V₀ of the capacitor C32 may be still lowerthan the reference voltage V_(T)−V_(x) even if the charges in thecapacitor C31 is transferred to the capacitor C32. In other words, thethird phase is not entered in the phase repetition shown in FIG. 4, andthus the pulse of the control signal V₁ is not generated. To preventthis state, in the second embodiment, a bias voltage by-pass unit forcompletely applying the bias voltage V_(T) to the capacitor C32 isprovided in the bias setting circuit.

FIG. 5 is a circuit diagram of a bias setting circuit of a rectifiercircuit according to a second embodiment. In FIG. 5, the same componentsas those in FIG. 1 are labeled by the same reference characters, andtherefore the components is not explained here.

The bias voltage by-pass unit is composed of NMOS transistors M41 andM42 in the bias setting circuit 300 shown in FIG. 5. The NMOS transistorM41 has a source connected to the positive terminal of the bias voltagesource 110 and a drain connected to one end (in the positive line) ofthe capacitor C32. The NMOS transistor M42 has a source connected to thenegative terminal of the bias voltage source 110 and a drain connectedto the other end (in the negative line) of the capacitor C32. The gatesof the NMOS transistors M41 and M42 are both connected to a resetterminal T300. A reset signal V_(CNT) is input to the reset terminalT300.

The operation of the bias setting circuit 300 will be described below.FIG. 6 is a-timing chart of the potential V₀ at one end of the capacitorC32, the differential voltage V_(E) output from the differentialamplifier 130, the control signal V₁ output from the inverter INV12, thesignal V₂ output from the inverter INV11, and the reset signal VCNT.

As shown in FIG. 6, the NMOS transistors M41 and M42 are both ON whenthe pulse of the reset signal VCNT is input, i.e., during a time periodwhen the reset signal VCNT is a logic “high” (period t0-t1), so that thebias voltage V_(T) supplied from the bias voltage source 110 is suppliedto the capacitor C32 not through the dummy switching circuit. Therefore,the potential V₀ of the capacitor C32 becomes equal to the bias voltageV_(T). After that, on the trailing edge of the pulse of the reset signalVCNT, i.e., when the reset signal VCNT is a logic “low”, the operationenters the first phase. Since the signal V₂ output from the inverterINV11 during a time period when the potential V₀ of the capacitor C32 isequal to the bias voltage V_(T) (period t₀-t₁), the NMOS transistors M32and M34 are both ON, so that the bias voltage V_(T) is also applied tothe capacitor C31. This means that the second and third phases followingthe first phase are executed.

The bias voltage by-pass unit as described above may be provided in notonly the bias setting circuit but also a rectifier circuit. FIG. 7 is acircuit diagram of the rectifier circuit of the rectifier circuitaccording to the second embodiment. In FIG. 7, the same components as inthe rectifier circuit 200 shown in FIG. 1 are labeled by the samereference characters, and therefore, the components is not explainedhere. The bias voltage by-pass unit is composed of NMOS transistors M51to M54 in the rectifier circuit 400 shown in FIG. 7. The NMOS transistorM51 has a source connected to the positive terminal of the bias voltagesource 210 and a drain connected to one end (in the first positive line)of the capacitor C12. The NMOS transistor M52 has a source connected tothe negative terminal of the bias voltage source 210 and a drainconnected to the other end (in the first negative line) of the capacitorC12. The NMOS transistor M53 has a source connected to the positiveterminal of the bias voltage source 210 and a drain connected to one end(in the second positive line) of the capacitor C22. The NMOS transistorM54 has a source connected to the negative terminal of the bias voltagesource 210 and a drain connected to the other end (in the secondnegative line) of the capacitor C22. Each gate of the NMOS transistorsM51 to M54 is connected to a reset terminal T400. The reset signalV_(CNT) is input to the reset terminal T400.

This bias voltage by-pass unit causes the rectifier circuit 400 tooperate in the initial stage according to the same timing chart shown inFIG. 6. Specifically, the bias voltage V_(T) supplied from the biasvoltage source 210 is applied to the capacitors C12 and C22 in therectification circuit and the capacitors C11 and C21 in the switchingcircuit by the reset signal V_(CNT).

The reset signal V_(CNT) can be generated through the power supplydetection at, for example, starting the rectifier circuit. FIG. 8 is acircuit diagram of a power detector circuit for generating the resetsignal V_(CNT). The power detector circuit 500 shown in FIG. 8 includesa capacitor C501, a first CMOS inverter composed of a PMOS transistorM501 and an NMOS transistor M502, a second CMOS inverter composed of anPMOS transistor M503 and an NMOS transistor M504, and a pull-downresistor R1. One end of the capacitor C501 and each source of the PMOStransistors M501 and M503 are all connected to a power supply lineV_(DD). Each drain of the NMOS transistors M502 and M504 is connected tothe ground. The other end of the capacitor C501 is connected to theinput terminal of the first CMOS inverter, i.e., each gate of the PMOStransistor M501 and the NMOS transistor M502. The output terminal of thefirst CMOS inverter, i.e., each drain of the PMOS transistor M501 andthe NMOS transistor M502 is connected to the input terminal of thesecond CMOS inverter, e.g., each gate of the PMOS transistor M503 andthe NMOS transistor M504. The output terminal of the second CMOSinverter, i.e., each drain of the PMOS transistor M503 and the NMOStransistor M504 is connected to an output terminal from which the resetsignal V_(CNT) is output. The pull-down resistor R1 is connected betweenthe gate of the NMOS transistor M502 and the ground.

The operation of the power detector circuit 500 will be described below.FIG. 9 is a timing chart of the potential V_(INT) at the other end ofthe capacitor C501 (potential at a node N1 in FIG. 8) and the resetsignal V_(CNT) to be generated. When a power supply voltage V_(DD) iscaused at the starting of the rectifier circuit, the capacitor C501 ischarged on the leading edge of the power supply voltage V_(DD) (untiltime t₁). The potential V_(INT) of the input terminal of the first CMOSinverter also rises during the leading, is accepted by the first CMOSinverter as a signal of logic “high” when reaching the threshold voltageV_(th) of the NMOS transistor M502 (time t₀). As a result, the firstCMOS inverter outputs a signal of logic “low”, and the second CMOSinverter outputs the reset signal V_(CNT) of logic “high”. After thecapacitor C501 has been charged (after time t₁), the potential V_(INT)gradually decreases to the ground potential through the pull-downresistor R1, and finally becomes lower than the threshold voltage V_(th)of the NMOS transistor M502 (time t₂). As a result, the first CMOSinverter outputs a signal of logic “high”, and the second CMOS inverteroutputs the reset signal of logic “low”. According to the aboveoperation, the power detector circuit 500 can generate the pulsed resetsignal V_(CNT) at the starting of the rectifier circuit.

As described above, the rectifier circuit according to the secondembodiment can apply the bias voltage VT as an initial value to thecapacitor C32 in the bias setting circuit 300 to be monitored and thecapacitor C31 repeatedly applied with the bias voltage V_(T).

A rectifier circuit according to a third embodiment includes a pluralityof rectifier circuits each corresponding to that in the first embodimentor the second embodiment, these rectifier circuits beingcascade-connected. The rectifier circuit, in particular, is foreliminating the disadvantages of the cascade connection of the rectifiercircuits. To begin with, the disadvantages will be explained below.Here, the rectifier circuit 200 in the first embodiment is taken as oneof the rectifier circuits (hereinafter, referred to as “cascaderectifier circuit”) to be cascade-connected

FIG. 10 is a circuit diagram of a cascade rectifier circuit. A cascaderectifier circuit 600 shown in FIG. 10 includes n rectifier circuitstages 200-1 to 200-n that are cascade-connected, a bias voltage source610, inverters INV1 and INV2, a backflow preventer circuit 630, and abattery 660. Each of the rectifier circuit stages 200-1 to 200-nincludes the rectification circuit and eight NMOS transistors thatfunction as transfer gates, out of the components of the rectifiercircuit 200 shown in FIG. 1. The positive terminal of one rectifiercircuit stage and the negative terminal of the other rectifier circuit,out of the adjacent rectifier circuit stages, are connected to eachother, and the transfer gates share two signal lines connected to thegates.

FIG. 10 shows a detailed circuitry of only a rectifier circuit stage200-1 that is the end of the rectifier circuit stages 200-1 to 200-n.NMOS transistors M1-1, M2-1, M11-1 to M14-1, and M21-1 to M24-1, andcapacitors C1-1, C2-1, C11-1, C12-1, C21-1, and C22-1 correspond to theNMOS transistors M1, M2, M11 to M14, and M21 to M24, and the capacitorsC1, C2, C11, C12, C21, and C22, respectively. The inverters INV1 andINV2 also are shared with the rectifier circuit stages 200-1 to 200-n,and function like the inverters INV1 and INV2 shown in FIG. 1.

The battery 660 is a power supply where a power-supply voltage V_(DD) isgenerated, and let it be a secondary cell here. E>>This battery 660 isconnected to the positive terminal of the rectification circuit in thehighest rectifier circuit stage 200-n through the backflow preventercircuit 630. Hence, The voltage rectified by the rectifier circuitstages 200-1 to 200-n is stored in the battery 660. The backflowpreventer circuit 630 is for example a diode whose cathode is connectedto the battery 660.

As shown in FIG. 10, since the NMOS transistors that constitute eachrectification circuit in the rectifier circuit stages 200-1 to 200-n areconnected in series in a line from the battery 660 to the ground,leakage current caused in each of the rectifier circuit stages is thesame. In other words, there is no deference in the leakage currentcaused in the rectification stages among the rectifier circuit stages200-1 to 200-n. Therefore, the leakage current in the rectificationcircuit caused in the cascade rectifier circuit 600 can be imitatedaccurately by the bias setting circuit 100 shown in FIG. 1.

Next, focus on the lowest rectifier circuit stages 200-1 in the cascaderectifier circuit 600. The NMOS transistors M11-1, M13-1, M21-1, andM23-1 are OFF if the voltage across each of the capacitors C12-1 andC22-1 is equal to the bias voltage V_(T) supplied from the bias voltagesource 610. In this situation, when a weak AC signal is input to aconnecting point between the NMOS transistors M1-1 and M2-1 in therectification circuit, the connecting point shows almost the samepotential as that at the source of the NMOS transistor M2-1, i.e.,almost the ground potential. Therefore, the potential at the gate of theNMOS transistor M1-1 is the same as the voltage V_(T) across thecapacitor C12-1.

Since the gate of the NMOS transistor M1-1 is connected to the drain ofthe NMOS transistor M11-1, which is a transfer gate, the drain of theNMOS transistor M11-1 shows the potential V_(T). The source of the NMOStransistor M11-1 shows a potential at the positive terminal of the biasvoltage source 610 through the NMOS transistor 12-1, which is turned ONby complementary operation with the NMOS transistor 12-1, i.e., the biasvoltage V_(T). Therefore, no potential difference occurs between thesource and drain of the NMOS transistor M11-1, and the leakage currentof the NMOS transistor M11-1 is vanishingly small.

In the same situation, the voltage between the source and drain ofrespective NMOS transistors M13-1, M21-1, and M23-1 is almost zero, andthe leakage current of these NMOS transistors is vanishingly small. Thesame is true of the rectifier circuit stage 200-n.

Next, focus the state of the highest rectifier circuit stage when a highpower signal is input and charges the bias voltage source 610. FIG. 11shows the same cascade rectifier circuit 600 as that in FIG. 10 but adetailed circuitry of the highest rectifier circuit stage 200-n, out ofthe rectifier circuit stages 200-1 to 200-n. NMOS transistors M1-n,M2-n, M11-n to M14-n, and M21-n to M24-n, capacitors C1-n, C2-n, C11-n,C12-n, C21-n, and C22-n in FIG. 11 correspond to the NMOS transistorsM1, M2, M11 to M14, and M21 to M24, and the capacitors C1, C2, C11, C12,C21, and C22 in FIG. 1, respectively.

The NMOS transistors M11-n, M13-n, M21-n, and M23-n are OFF if thevoltage across each of the capacitors C12-n and C22-n is equal to thebias voltage V_(T) supplied from the bias voltage source 610. In thissituation, when a high power signal, which is able to charge the biasvoltage source 610, is input to the signal input terminal TA, i.e., aconnecting point between the NMOS transistors M1-n and M2-n in therectification circuit, the connecting point shows almost the samepotential as the sum of the voltages across of the smoothing capacitorsC2-1 to C2-(n-1) in the rectifier circuit stages 200-1 to 200-(n-1).Since the voltage across the input and output terminals of the backflowpreventer circuit 630 is few, the potential at the connecting point isalmost the power supply voltage VDD for charging the battery 660.Therefore, the potential at the gate of the NMOS transistor M1-n showsV_(T)+V_(DD) calculated by adding the power supply voltage V_(DD) to thevoltage VT across the capacitor C12-n.

Since the gate of the NMOS transistor M1-n is connected to the drain ofthe NMOS transistor M11-n, which is a transfer gate, the drain of theNMOS transistor M11-n shows the potential V_(T)+V_(DD). The source ofthe NMOS transistor M11-n shows the same potential as that at thepositive terminal of the bias voltage source 610, i.e., the bias voltageV_(T), through the NMOS transistor 12-n, which is turned ON by thecomplementary operation with the NMOS transistor M11-n. Therefore, thepotential difference V_(DD) causes between the source and drain of theNMOS transistor M11-n. The potential difference V_(DD) causes thecharges in the capacitor C12-n to move to the capacitor C11-n. In thesame situation, the voltage between the source and drain of the NMOStransistor M13-n is also almost V_(DD), and thus leakage current flowstherethrough. Therefore, the capacitor C12-n connected to the NMOStransistor M1-n in the rectification circuit is discharged with theleakage current of the NMOS transistor M11-n, which is a transfer gate.For similar reasons, the capacitor C22-n connected to the NMOStransistor M2-n in the rectification circuit is discharged with theleakage current of the NMOS transistor M21-n, which is a transfer gate.

However, the bias setting circuit 100 shown in FIG. 1 does not imitatethe generation of leakage current of these transfer gates. In otherwords, the bias setting circuit 100 cannot accurately monitor the stateof potential of the capacitor in the rectification circuit of thecascade rectifier circuit 600 shown in FIG. 10. The rectifier circuitaccording to the third embodiment can eliminate this disadvantages.

FIG. 12 is a circuit diagram of a bias setting circuit of the rectifiercircuit according to the third embodiment. In FIG. 12, the samecomponents as those in the bias setting circuit 100 of FIG. 1 arelabeled by the same reference characters, and therefore, the componentsis not explained here.

The bias setting circuit 700 shown in FIG. 12 differs from the biassetting circuit 100 of FIG. 1 in that (i) two NMOS transistors M61 andM62 and a current source 710 are provided, (ii) a reference voltagesource 720 connected to the inverting input terminal IN2 of thedifferential amplifier 130 generates a reference voltageV_(DD)−V_(T)−V_(X), and (iii) the drain and source of the NMOStransistor M30 in the dummy rectification circuit is not grounded.

Each of the NMOS transistors M61 and M62 has a gate and a drain whichare connected to each other, and functions as a load element. Referringto FIG. 12, the source of the NMOS transistor M61 and the drain of theNMOS transistor M62 are connected to each other. The drain of the NMOStransistor M61 is connected to a power supply line V_(DD), and thesource of the NMOS transistor M62 is connected to the drain and sourceof the NMOS transistor M30. The current source 710 is connected to thedrain and source of the NMOS transistor M30. In this circuitry, the NMOStransistors M61 and M62 causes almost the same voltage drop as the sumof the threshold voltages of the NMOS transistors M61 and M62 betweenthe power supply line and the NMOS transistor M30 with weak currentsupplied from the current source 710. Each of the NMOS transistors M61and M62 has a threshold voltage equal to the bias voltage V_(T)generated by the bias voltage sources 110 and 210. Therefore, apotential V_(DD)−2V_(T) is applied to the drain and source of the NMOStransistor M30. A general bias circuit may be used as the current source710, or be composed of a transistor whose gate-to-source voltage is setto 0 V for using leakage current thereof.

In the state where the voltage across the capacitor C32 is equal to thebias voltage V_(T), the drain of the NMOS transistor M31, which is atransfer gate, shows a potential V_(DD)−V_(T) calculated by adding thebias voltage V_(T) to a potential V_(DD)−2V_(T) at the drain and sourceof the NMOS transistor M30. The source of the NMOS transistor M31 showsthe same potential as that at the positive terminal of the bias voltagesource 110, i.e., the bias voltage V_(T), through the NMOS transistorM32, which is turned ON by the complementary operation with the NMOStransistor M31. Therefore, the potential difference V_(DD)−2V_(T) occursbetween the source and drain of the NMOS transistor M31. The potentialdifference V_(DD)−2V_(T) causes the capacitor C32 to be discharged. Inother words, the state of current leakage similar to that in the highestrectifier circuit stage 200-n shown in FIG. 11 is presented. However,the source-to-drain voltage V_(DD)−2V_(T) of the NMOS transistor M31 issmaller than the source-to-drain voltage V_(DD) of the NMOS transistorM11-n in the rectifier circuit stage 200-n by 2V_(T). Therefore, thestate of current leakage of the cascade rectifier circuit 600 is notimitated accurately by the bias setting circuit 700. However, thedifference of 2V_(T) can be disregarded by adjustment the scale of theNMOS transistor M31. In general, the leakage current of the NMOStransistor whose gate is grounded depends on the drain-to-source voltage(V_(DS)). The dependency is decided according to the process conditionof the transistor. If the V_(DS) dependence of the NMOS transistor M31is small, the leakage current caused by V_(DS) is small. If the V_(DS)dependence of the NMOS transistor M31 is large, the leakage currentchanges by a factor of several depending on the differential voltage2V_(T). In view of this, the scale of the NMOS transistors M31 and M33is designed one to ten times as large as each transfer gate in thecascade rectifier circuit 600. As a result, the state of electriccurrent leakage in the cascade rectifier circuit 600 can be imitated inthe bias setting circuit 700.

The operation of the bias setting circuit 700 follows the timing chartshown in FIG. 4, except that the reference voltage to be compared withthe potential V₀ of the capacitor C32 is V_(DD)−V_(T)−V_(X), and is thusnot explained here.

There is a possibility that leakage current occurs in the transfer gatesin the rectifier circuit stages 200-2 to 200-(n-1) other than thehighest rectifier circuit stage 200-n. However, the most leakage currentoccurs in the highest rectifier circuit stage 200-n. Consequently, it iseffective for the bias setting circuit 700 to imitate the generation ofthe drain-to-source voltage V_(DD) of the transfer gate in the highestrectifier current circuit.

As described above, in the rectifier circuit according to the thirdembodiment, the bias setting circuit 700 can imitate the state ofcurrent leakage of a transfer gate in the highest rectifier circuitstage 200-n of the cascade rectifier circuit 600. As a result, it ispossible for the cascade rectifier circuit 600 to have the sameadvantages as that in the first embodiment.

A rectifier circuit according to a fourth embodiment includes a biassetting circuit for eliminating the disadvantages described in the thirdembodiment in another way. FIG. 13 is a circuit diagram of a biassetting circuit of the rectifier circuit according to the fourthembodiment. In FIG. 13, the same components as those in the bias settingcircuit shown in FIG. 9 are labeled by the same reference characters,and therefore, the components is not explained here.

The bias setting circuit 800 shown in FIG. 13 defers from the biassetting circuit 100 shown in FIG. 1 in that (i) a PMOS transistor M71,and NMOS transistors M73 and M72, which are connected in series, acurrent source 810, a delay circuit 830, an EXOR circuit 840, and an ANDcircuit 850, (ii) a reference voltage source 820 connected to theinverting input terminal IN2 of the differential amplifier 130 generatesa reference voltage V_(DD)−V_(X), and (iii) the drain and source of theNMOS transistor M30 in the dummy rectification circuit is not grounded.

The combination of the PMOS transistor M71 and the NMOS transistor M72functions as a CMOS inverter. Referring to FIG. 13, the source of thePMOS transistor M71 is connected to a power supply line V_(DD), and thesource of the NMOS transistor M72 is connected to the ground. The gateand drain of the NMOS transistor M73 are both connected to the drain ofthe PMOS transistor M71, and the source of the NMOS transistor M73 isconnected to the drain of the NMOS transistor M72. A connecting pointbetween the NMOS transistors M72 and M73 is connected to the drain andsource of the NMOS transistor M30 in the dummy rectification circuit,and functions as an output node of the CMOS inverter. The gate of thePMOS transistor M71 and the gate of the NMOS transistor M72 areconnected to each other, and functions as an input node of the CMOSinverter.

The input node of the CMOS inverter is connected to the output terminalof the AND circuit 850 (a signal V_(c) is output), and the AND circuit850 receives the output (i.e., the control signal V₁) of the inverterINV12 and the output (a signal V_(b)) of the EXOR circuit 840. The EXORcircuit 840 receives the output of the inverter INV12 and the output (asignal V_(a)) of the delay circuit 830. The delay circuit 830 delays theoutput of the inverter INV12 by a time τ, thereby outputting the signalV_(a).

The operation of the bias setting circuit 800 will be described below.FIG. 14 is a timing chart of the control signal V₁ output from theinverter INV12, the signal V_(a) output from the delay circuit 830, thesignal V_(b) output from the EXOR circuit 840, and the signal V_(c)output from the AND circuit 850, the potential V₀ at one end of thecapacitor C32, and a potential V₃ at the other end of the capacitor C32.

Suppose an initial state immediately after the control signal V₁ entersa logic “high” from a logic “low”. In this state, the delay circuit 830starts counting the time τ. The output signal V_(a) is a logic “low”until the time τ elapses. Accordingly, as shown in FIG. 14, the signalsV_(b) and V_(c) are both logic “high” until the time τ elapses after thecontrol signal V1 enters a logic “high” from a logic “low”. Therefore, asignal of logic “high” is input to the gates of the PMOS transistor M71and the NMOS transistor M72, and the invert of the signal, a signal oflogic “low” is output from a connecting point between the NMOStransistor M72 and the PMOS transistor M73. Since this signal of logic“low” is almost 0 V (ground potential), the potential V3 at the otherend of the capacitor C32 is the ground potential. Meanwhile, thepotential V₀ at one end of the capacitor C32 is equal to a level higherthan the potential V₃ by the bias voltage V_(T), i.e., the potentialV_(T).

The time τ is at least a time required to have transferred charges fromthe capacitor C31 to the capacitor C32 through the NMOS transistors M31and M33. When the time τ elapses, the output signal V_(a) of the delaycircuit 830 is a logic “high”. The signal V_(b) remains at a logic“high” but the signal V_(c) is a logic “low”. A signal of logic “low” isinput to the gates of the PMOS transistor M71 and the NMOS transistorM72, the invert of the signal, a signal of logic “high” is output from aconnecting point between the NMOS transistors M72 and M73. This signalof logic “high” accurately shows a potential V_(DD)−V_(T) calculated bysubtracting the threshold voltage V_(T) of the NMOS transistor M73 fromthe power supply voltage V_(DD). Specifically, the potential V₃ of theother end of the capacitor C32 also shows V_(DD)−V_(T). Meanwhile, thepotential V₀ at one end of the capacitor C32 is equal to a level higherthan the potential V₃ by the bias voltage V_(T), i.e., the potentialV_(DD). The differential amplifier 130 outputs a differential voltageV_(X) between the potential V_(DD) and the reference voltageV_(DD)−V_(X). This differential voltage V_(X) is accepted by theinverter INV12 as a logic “high” as in the timing chart shown in FIG. 4,so that the control signal V₁ changes from a logic “high” to a logic“low”. The control signal V₁ of logic “low” turns OFF the NMOStransistors M31 and M33, and turns ON the NMOS transistors M32 and M34through the inverter INV11. In this state, since the potential V₀ isequal to the power supply voltage V_(DD), the drain-to-source voltage ofthe NMOS transistor M31 becomes V_(DD)−V_(T), so that the state ofcurrent leakage of the NMOS transistor M31 occurs. The potential V₀ ofthe capacitor C32 decreases gradually due to the leakage current andfinally becomes smaller than the reference voltage V_(DD)−V_(X).Specifically, the differential voltage V_(E) output from thedifferential amplifier 130 becomes an input signal of logic “low” forthe inverter INV12. As a result, the control signal V₁ shows a logic“high” again as in the initial state. After that, the phases arerepeated.

Consequently, the state of the capacitor in the rectification circuit ofthe rectifier circuit can be observed as the first embodiment. Inparticular, according to the fourth embodiment, the drain-to-sourcevoltage V_(DD)−V_(X) causes the state of current leakage due to the NMOStransistor M31. This voltage is closer to V_(DD) by the voltage V_(X)than the drain-to-source voltage V_(DD)−2V_(X) described in the thirdembodiment. In other words, this is a value closer to thedrain-to-source voltage V_(DD), which causes the leakage current in thehighest rectifier circuit stage in the cascade rectifier circuit, and itis therefore possible to imitate the state of current leakage in thecascade rectifier circuit 600 more accurately than the bias settingcircuit 700 of the third embodiment.

A rectifier circuit according to a fifth embodiment includes a biassetting circuit for eliminating the disadvantages described in the thirdembodiment in still another way. FIG. 15 is a circuit diagram of a biassetting circuit of a rectifier circuit according to a fifth embodiment.In FIG. 15, the same components as those in the bias setting circuit 100shown in FIG. 1 are labeled by the same reference characters, andtherefore, the components is not explained here.

The bias setting circuit 900 differs from the bias setting circuit 100shown in FIG. 1 in that (i) an NMOS transistor M81 is provided, (ii) oneend of the capacitor C32 (potential V₀) is connected to the invertinginput terminal IN2 of the differential amplifier 130, (iii) the positiveterminal of the bias voltage source 110 is connected to thenon-inverting input terminal IN1 of the differential amplifier 130, and(iv) no reference voltage source 120 is provided.

The drain of the NMOS transistor M81 is connected to a power supply line(power supply voltage V_(DD)), the source of the NMOS transistor M81 isconnected to one end of the capacitor C32 (potential V₀), and the gateof the NMOS transistor M81 is connected to the drain and source of theNMOS transistor M30.

In this circuitry, the NMOS transistor M81 is always OFF because thegate is grounded, and the drain-to-source voltage shows V_(DD)−V_(T)calculated by subtracting the threshold voltage V_(T) from the powersupply voltage V_(DD). This means leakage current of the NMOS transistorM81 caused by the drain-to-source voltage V_(DD)−V_(T) is supplied toone end (potential V₀) of the capacitor C32 during a time period whenthe NMOS transistors M31 and M33 are OFF. In other words, thedrain-to-source voltage V_(DD)−V_(T) of the NMOS transistor M31described in the fourth embodiment is generated with the NMOS transistorM81. However, the leakage current does not induce discharging thecapacitor C32 but rising the potential V₀ at one end of the capacitorC32. In view of this, the differential amplifier 130 detects an increasein the potential V₀ by the acceptance of the voltage V_(T) of the biasvoltage source 110 at the non-inverting input terminal IN1. Concretely,for example, the logical level of the differential voltage V_(E)reverses when the potential V₀ showing the bias voltage V_(T) increasesby more than the voltage V_(X), that is, when the potential V₀ becomeshigher than V_(DD)−V_(T). The operation of the bias setting circuit 900follows the timing chart shown in FIG. 4 otherwise, and is thus notexplained here. The voltage V_(X) is obtained by adjustment of the scaleof the NMOS transistors M123 and M124 in FIG. 2.

As described above, in the rectifier circuit according to the fifthembodiment, the bias setting circuit 900 can imitate the state ofcurrent leakage of a transfer gate in the highest rectifier circuitstage 200-n of the cascade rectifier circuit 600. As a result, it ispossible for the cascade rectifier circuit 900 to have the sameadvantages as that in the first embodiment.

A rectifier circuit according to a sixth embodiment is a modification tothe rectifier circuit according to the fifth embodiment. FIG. 16 is acircuit diagram of a bias setting circuit of the rectifier circuitaccording to the sixth embodiment. In FIG. 16, the same components asthose in the bias setting circuit 100 shown in FIG. 9 are labeled by thesame reference characters, and therefore, the components is notexplained here.

The bias setting circuit 1000 shown in FIG. 16 differs from the biassetting circuit 100 shown in FIG. 1 in that a second dummy rectificationcircuit and a second dummy switching circuit are added. Hereafter, thedummy rectification circuit and the dummy switching circuitcorresponding to those in FIG. 1 are referred to as “a first dummyrectification circuit” and “a first dummy switching circuit”,respectively. The bias setting circuit 1000 includes an NMOS transistorM30′ and a capacitor C32′ corresponding to the NMOS transistor M30 andthe capacitor C32 in the first dummy rectification circuit,respectively. The NMOS transistor M30′ and the capacitor C32′ constitutethe second dummy rectification circuit. The bias setting circuit 1000also includes NMOS transistors M31′, M32′, M33′, and M34′, a capacitorC31′, and a bias voltage source 110′ corresponding to the NMOStransistors M31, M32, M33, and M34, the capacitor C31, and the biasvoltage source 110 in the first dummy rectification circuit,respectively. The NMOS transistors M31′ to M34′, the capacitor C31′, andthe bias voltage source 110′ constitute the second dummy switchingcircuit. The bias setting circuit 1000 further differs from the biassetting circuit 100 in that (i) an NMOS transistor M91 is provided, (ii)one end (potential V₀′) of the capacitor C32′ is connected to theinverting input terminal of the differential amplifier 130, and (iii) noreference voltage source 120 is provided.

The drain of the NMOS transistor M91 is connected to a power supply line(power-supply voltage V_(DD)), and the source of the NMOS transistor M91is connected to one end (potential V0′) of the capacitor C32′, and thegate of the NMOS transistor M91 is connected to the drain and source ofthe NMOS transistor M30′.

The bias setting circuit 1000, in other words, has a circuitry where thenon-inverting input terminal IN1 of the differential amplifier 130 isconnected to one end (potential V₀) of the capacitor C32 in the firstdummy rectification circuit instead of the positive terminal of the biasvoltage source 110 in FIG. 15.

As a result, the operation of the bias setting circuit 1000 is almostthe same as that of the bias setting circuit 900 described in the fifthembodiment. The bias setting circuit 1000, however, has an advantagethat a differential voltage due to leakage current of the capacitor C32′can be detected more accurately because the reference voltage input tothe non-inverting input terminal of the differential amplifier 130 isnot the bias voltage V_(T) but the present potential of the capacitorC32 in the rectification circuit.

The rectifier circuit according to any one of the first to sixthembodiments can be used as a rectifier circuit for an RFID tag.

A seventh embodiment is to describe an RFID tag using the rectifiercircuit according to the second embodiment. FIG. 17 is a circuit diagramof the RFID tag according to the seventh embodiment. An RFID tag 1010shown in FIG. 17 includes an antenna 9, a bias setting circuit 10 asshown in FIG. 5, a rectifier circuit 20 as shown in FIG. 7, a backflowpreventer circuit 30, an RF detector circuit 40, a control circuit 50,and a battery 60 that is a secondary cell. Here, the bias settingcircuit 10 and the rectifier circuit 20 constitute the rectifier circuitaccording to the second embodiment. This RFID tag 1010 is operated by apower supply voltage VDD supplied from the battery 60, and it is notalways necessary to generate a power supply voltage from the rectifiercircuit 20 for its operation. Specifically, the bias setting circuit 10,the rectifier circuit 20, the backflow preventer circuit 30, the RFdetector circuit 40, and the control circuit 50 are all connected to apower supply line and a ground line which extend from the battery 60. Inthis embodiment, suppose that the reset signal VCNT is not generated bythe power detector circuit shown in FIG. 8 but by the control circuit50.

The antenna 9 induces an alternating current in its antenna lineaccording to magnetic flux variation generated by a reader and writer(not shown in the figure). This alternating current is input to thesignal input terminal of the rectifier circuit 20. The rectifier circuit20 operates at the power supply voltage supplied from the battery 60.Therefore, the bias voltage source or the reference voltage source asshown in FIGS. 3A to 3D generates the bias voltage V_(T) or thereference voltage V_(T)−V_(X) from the power supply voltage V_(DD)supplied from the battery 60. Specifically, the bias voltage V_(T) isapplied between the gate and source of the MOS transistors constitutingthe rectification circuit in the rectifier circuit 20, regardless ofwhether an alternating current is supplied from the antenna 9.Therefore, the rectifier circuit 20 can rectify a weak alternatingcurrent induced in the antenna 9 with a root-mean-square value of lessthan approximately 0.7 V, as described in the first and secondembodiments. In other words, it is possible to demodulate the weak datasignal received by the antenna 9. For rectification, the control signalV₁ is input from the bias setting circuit 10 to the rectifier circuit 20at the perfect time, and the rectifier circuit 20 maintains the biasvoltage of the MOS transistor constituting the rectification circuit atmore than a predetermined value based on the control signal V₁.

The demodulated data signal is transmitted to the RF detector circuit40. The RF detector circuit 40 detects the data signal to drive thecontrol circuit 50 and to output the detected data signal to the controlcircuit 50. A DC voltage obtained by the rectifier circuit 20 issupplied to the battery 60 as an electric power for charge through thebackflow preventer circuit 30.

The control circuit 50 reads out data stored in a memory (not shown inthe figure) based on the data signal received from the rectifier circuit20 and writes data in the memory. The stored data is, for example, tagidentification information. The control circuit 50 includes a loadmodulating unit connected to the antenna 9. The data read out from thememory is transmitted to the reader and writer by modulating currentflowing in the antenna 9 with the load modulating unit. Concretely, theload modulation unit generates a demagnetizing field in the antenna 9.The demagnetizing field makes a slight change in the current that flowsin the reader and writer's antenna. This slight change is detected bythe reader and writer, and identified as a data signal.

Moreover, the control circuit 50 generates the reset signal V_(CNT)input to the bias setting circuit 10 and the rectifier circuit 20. FIG.18 is a timing chart of the reset signal V_(CNT). As shown in FIG. 18,the control circuit 50 outputs a pulse of the reset signal V_(CNT)immediately before entering suspend stage from running state (time t₁)so that the bias setting circuit 10 and the rectifier circuit 20 areinitialized. The reset signal V_(CNT) is not generated at the timingaccording to FIG. 18 because the control circuit 50 does not run yetwhen the battery 60 is first installed in the RFID tag after the RFIDtag is manufactured. However, it is possible to run the control circuit50 by transmitting a high power RF signal to the RFID tag onmanufacturing test, so that the reset signal V_(CNT) is generated. Inother words, the circuitry composed of the bias setting circuit 10 andthe rectifier circuit 20 does not run as a rectifier circuit but as aconventional low gain rectifier circuit before manufacturing test.Applying a high power RF signal makes the circuitry be a rectifiercircuit.

The reset signal V_(CNT) may be generated by the power detector circuitshown in FIG. 8 instead of the control circuit 50. FIG. 19 is a blockdiagram of an RFID tag with a power detector circuit. In FIG. 19, thesame components as those in the RFID tag shown in FIG. 17 are labeled bythe same reference characters, and therefore, the components is notexplained here. In the RFID tag 1020 tag shown in FIG. 19, a powerdetector circuit 70 as shown in FIG. 8 is installed, and the powerdetector circuit 70 generates the reset signal V_(CNT) instead of thecontrol circuit 50. As a result, even when the battery 60 is firstinstalled in the RFID tag after the RFID tag is manufactured, the powerdetector circuit 70 detects that, so that the reset signal V_(CNT) isgenerated.

Moreover, since the RFID tag according to this embodiment includes thebattery 60, it is easy to install various input-output devices, such asa temperature sensor, a speaker, a microphone, and a light emittingdevice, in the RFID tag. Such an RFID tag has a broader application. AnRFID tag with a sensor has a structure shown in FIG. 20 for example. InFIG. 20, the same components as those in the RFID tag shown in FIG. 17are labeled by the same reference characters, and therefore, thecomponents is not explained here. In an RFID tag 1030 shown in FIG. 20,a sensor 80 is connected to a power supply line and a ground line whichare extended from the battery 60. The control circuit 50 transmits andreceives signals to and from the sensor 80. As an example of the sensorinstalled in the RFID tag, a temperature sensor will now be explained.The temperature sensor is in sleep and does not use power during notransmission of signal from a reader and writer (not shown in thefigure). When the control circuit 50 sends a request to the RFID tagwith the temperature sensor based on a signal transmitted by the readerand writer, the temperature sensor is activated to detect temperatureand then to transmit temperature data to the control circuit 50. Thistemperature data and unique data of the RFID tag are transmitted fromthe RFID tag to the reader and writer. As another operation of thetemperature sensor, the control circuit 50 may send a request for outputof temperature data to the temperature sensor at given time intervals tostore the temperature data in a memory (not shown in the figure). And,the control circuit 50, when receiving a request from the reader andwriter, transmits the stored temperature data together with detectiontime data to the reader and writer. The temperature sensor may beactivated by a trigger such as vibration, sound, and light to store thetemperature data in the memory.

According to the RFID tag according to the seventh embodiment asdescribed above, since the rectifier circuit according to the secondembodiment is provided, it is possible to prevent overcharge for biasingthe MOS transistors constituting the rectification circuit in therectification circuit and thus to reduce power consumption.

Moreover, it is possible to provide a rectification gain more than apredetermined value without influence of manufacturing differences inthe rectifier circuits.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A rectifier circuit comprising: a first MOS transistor; a firstcapacitor configured to connect between a gate and a source of the firstMOS transistor; a switching circuit configured to supply a bias voltageto the first capacitor in response to a control signal; a second MOStransistor configured to imitate the first MOS transistor; a secondcapacitor configured to imitate the first capacitor; a dummy switchingcircuit configured to supply the bias voltage to the second capacitor inresponse to the control signal; and a generating circuit configured togenerate the control signal based on a potential of the secondcapacitor.
 2. The rectifier circuit according to claim 1, furthercomprising a bias voltage source configured to include a third MOStransistor, and generates the bias voltage from a gate-to-source voltageof the third MOS transistor.
 3. The rectifier circuit according to claim1, further comprising a reference voltage source configured to generatea reference voltage lower than the bias voltage, wherein the biassetting circuit generates the control signal based on a result ofcomparison of the reference voltage and the potential of the secondcapacitor.
 4. The rectifier circuit according to claim 3, wherein thereference voltage source includes a fourth MOS transistor, and generatesthe reference voltage from a gate-to-source voltage of the fourth MOStransistor.
 5. The rectifier circuit according to claim 1, wherein thebias setting circuit supplies the bias voltage to the second capacitorin response to a reset signal for initializing a voltage across thesecond capacitor.
 6. The rectifier circuit according to claim 5, whereinthe rectifier circuit supplies the bias voltage to the first capacitorin response to the reset signal.
 7. The rectifier circuit according toclaim 5, further comprising a power detector circuit configured togenerate the reset signal in response to detection of a power supplyvoltage for the rectifier circuit.
 8. The rectifier circuit according toclaim 1, wherein a source of the second MOS transistor is grounded. 9.The rectifier circuit according to claim 1, wherein a source of thesecond MOS transistor has a potential between a power supply potentialand the ground potential.
 10. The rectifier circuit according to claim1, further comprising: a logic circuit configured to generate a logicsignal based on the control signal and a signal delayed by apredetermined time to the control signal; and an inverter circuitconfigured to supply one of the ground potential and a potential betweena power supply potential and the ground potential to a source of thesecond MOS transistor in response to the logic signal.
 11. The rectifiercircuit according to claim 10, wherein the predetermined time is a timerequired to have supplied the bias voltage to the second capacitorthrough the dummy switching circuit.
 12. The rectifier circuit accordingto claim 1, wherein the bias setting circuit further includes a currentsupply circuit for supplying a predetermined current to one end of thesecond capacitor, and generates the control signal based on a potentialof the second capacitor increased with the predetermined current flowingtherein.
 13. The rectifier circuit according to claim 12, wherein theswitching circuit includes a MOS transistor for supplying the biasvoltage to the first capacitor in response to the control signal, andthe predetermined current corresponds to a leakage current of the MOStransistor in the switching circuit.
 14. The rectifier circuit accordingto claim 12, wherein the bias setting circuit further includes a thirdMOS transistor imitating the first transistor, a third capacitorimitating the first capacitor, and a second dummy switching circuit forsupplying the bias voltage to the third capacitor in response to thecontrol signal, and generates the control signal based on potentials ofthe second capacitor and the third capacitor.
 15. The rectifier circuitaccording to claim 1, wherein the switching circuit includes a fifth MOStransistor for supplying the bias voltage to the first capacitor inresponse to the control signal, and the dummy switching circuit includesa sixth MOS transistor imitating the fifth MOS transistor and beinglarger in scale than the fifth MOS transistor.
 16. A radio frequencyidentification tag comprising: an antenna; a first MOS transistor; afirst capacitor configured to connect between a gate and a source of thefirst MOS transistor; a switching circuit configured to supply a biasvoltage to the first capacitor in response to a control signal; a secondMOS transistor configured to imitate the first MOS transistor; a secondcapacitor configured to imitate the first capacitor; a dummy switchingcircuit configured to supply the bias voltage to the second capacitor inresponse to the control signal; generating unit configured to generatethe control signal based on a potential of the second capacitor; abattery configured to be charged with a direct current supplied from therectifier circuit; and a control circuit configured to transmit tagidentification information via the antenna based on the direct current.17. The radio frequency identification tag according to the claim 16,further comprising a sensor, wherein the control circuit transmits asignal output from the sensor via the antenna.
 18. A radio frequencyidentification tag comprising: an antenna; a first MOS transistor; afirst capacitor configured to connect between a gate and a source of thefirst MOS transistor; a switching circuit configured to supply a biasvoltage to the first capacitor in response to a control signal; a secondMOS transistor configured to imitate the first MOS transistor; a secondcapacitor configured to imitate the first capacitor; a dummy switchingcircuit configured to supply the bias voltage to the second capacitor inresponse to the control signal; a generating unit configured to generatethe control signal based on a potential of the second capacitor asupplying unit configured to supply the bias voltage to the secondcapacitor in response to a reset signal; a battery configured to becharged with a direct current supplied from the rectifier circuit; and acontrol circuit configured to transmit tag identification informationvia the antenna based on the direct current, and outputs the resetsignal when the control circuit enters suspend state from running state.19. The radio frequency identification tag according to the claim 18,further comprising a sensor, wherein the control circuit transmits asignal output from the sensor via the antenna.